1. Field of the Invention
The present invention relates to a positioning control system and a positioning control method which perform positioning control of a head, for example in a magnetic disk device.
2. Related Art
In a head positioning control system of a magnetic disk device, a periodic disturbance which is caused by eccentricity of a disk, or the like, and which is synchronized with the rotation frequency of the disk is added to a head position error signal (signal representing an error between a target head position and a present head position) obtained by reproducing data including in a servo sector. Thus, when the periodic disturbance is not suppressed, it is difficult to perform highly precise positioning.
The head positioning control system of the magnetic disk device in recent years is configured by a digital control system using a microcomputer, and hence it is effective to perform repetitive control by integrating an internal model (repetitive controller) corresponding to a rotation period length of the magnetic disk into a feedback loop of the head positioning control system. The repetitive controller is configured to improve the head positioning accuracy by suppressing the periodic disturbance in such a way that the position error signal which is inputted into the head positioning control system is recorded in correspondence with the rotation period length, and that the recorded signal is added to the position error signal at the corresponding time of the next rotation period.
However, when a non-periodic disturbance signal is inputted into the above described repetitive controller, there arises a state where the position error signal of the next period has no object to be suppressed. As a result, the output of the repetitive controller becomes a disturbance source which deteriorates the performance of the head positioning control system.
For example, there is considered a two-degree-of-freedom control system in which a same feedback controller is used for the seek control for moving a head to a target track, and is also used for tracking control for positioning the head at the center of the target track. The two-degree-of-freedom control system is generally used in a short distance seek mode in which the number of moving tracks at the time of seeking is comparatively small. In this case, when the above described repetitive controller records a transient position error signal (overshoot, undershoot, and the like) generated at the time when the seek control is completed, the recorded transient position error signal becomes a non-periodic disturbance. As a result, the above described problem of deterioration in the performance of the head positioning control system is caused due to reverse effect phenomenon of repetitive control after the system is shifted to the tracking control.